From commonly owned German patent specifications Nos. 1,261,360 and 1,625,849 it is known to provide a self-actuating clutch for the purpose of temporarily coupling a motor-driven shaft with a transmission member such as a spur gear freely rotatable on that shaft, the clutch including a ring with a beveled inner friction surface coacting with a complementarily beveled friction surface on the gear hub for rotatively entraining same when the ring is axially shifted from a normal disengaged position toward the gear. Such a shift is brought about by a control sleeve which is slidably but nonrotatable mounted on a generally cylindrical supporting body rigid with the shaft, this body being provided with a set of angularly equispaced radial pins carrying rollers which engage generally V-shaped sections of a cam track on a face of the clutch ring remote from the associated transmission member. The control sleeve has its inner peripheral surface formed with a set of angularly equispaced, peripherally extending grooves engaged by respective studs rising from the clutch ring to enable displacement of that ring by an axial shift of the sleeve; and clutch ring is limitedly rotatable relatively to the sleeve-supporting body but is normally held in a predetermined angular position with reference to that body by the V-shaped cam-track sections and a set of springs axially drawing the ring toward the body. The spring force also tends to hold the control sleeve in a neutral position from which it can be axially shifted to engage the clutch by establishing contact between the two coacting friction surfaces. This contact need only suffice to exert a certain drag upon the clutch ring (it being assumed that the shaft rotates faster than the gear) so as to cause a limited rotation of the ring relative to the sleeve support. The shape of the cam track translates this relative rotation into a further axial shift of the ring, proportional to the angle of rotation, which intensifies the frictional engagement and therefore the transmitted torque.
In the case of a double-clutch assembly, in which the sleeve support is bracketed by two clutch rings confronting respective transmission members, the grooves of the sleeve are duplicated at opposite ends thereof to receive respective sets of studs from the two clutch rings. In that case it is necessary to provide these grooves with a spur adapted to receive the associated stud in the normal angular position of the corresponding ring to allow a shifting of the sleeve in the opposite axial direction. To facilitate bidirectional rotation, the grooves are substantially T-shaped with the spur consisting the stem of the "T".
In this known system each stud fits closely into the associated groove, or at least into the peripherally extending leg thereof, so that the sleeve fully participates in the axial excursion of the engaged clutch ring. To disengage the clutch constituted by this ring and the coacting gear hub, the sleeve must be brought bact to its neutral position by a reverse axial shift which has to overcome the reaction force of the cam track and the associated rollers on the sleeve support as long as the ring is in an off-normal angular position relative to that support. The reaction force to be overcome is a function of the driving torque applied to the shaft so that disengagement of the clutch is possible only with low or zero torque, i.e. with the motor sharply braked or disconnected from the shaft. This is true even where, as in the system described in the second one of the two German patent specifications referred to above, the gears to be entrained are axially shiftable on the shaft in a direction away from the sleeve support, against the force of prestressed retaining springs which limit the contact pressure between the coacting friction surfaces of the engaged clutch.